1. Field of the Invention
The present invention relates to a thin film transistor (TFT), and more particularly, to a method of manufacturing the same. Also, the present invention relates to an active matrix display device and a manufacturing method thereof.
2. Description of the Related Art
As a type of flat panel display device, an organic electro-luminesence (EL) display device is being watched with keener interest than any other display device, such as a cathode ray tube (CRT) and a liquid crystal display (LCD) device. In comparison to the CRT having the same screen size, the organic EL display device is thin, lightweight, and has lower power consumption. Since the organic EL display device emits light by itself, it does not require a back light device. Therefore, a lightweight, small-sized and compact display device can be achieved. In addition, the organic EL display device has an advantage in that there is no limitation to a viewing angle. As such, organic EL display device having a thin film transistor (TFT) as a switching element is being actively developed.
FIG. 1 is a cross-sectional view illustrating a conventional TFT. A process of manufacturing the conventional TFT is described below with reference to FIG. 1. First, a buffer layer 11 is formed on a transparent insulating substrate (“substrate”) 10. The substrate 10 is a transparent glass substrate or a transparent plastic substrate. A polycrystalline silicon layer is deposited on the buffer layer 11 and then patterned to form a semiconductor layer 12.
Then a first insulating layer 13 is deposited over the Whole surface of the substrate 10 as well as covering the semiconductor layer 12. The first insulating layer 13 serves as a gate insulating layer. A first metal layer is deposited on the first insulating layer 13 over the semiconductor layer 12 and then patterned to form a gate electrode 14. Using the gate electrode 14 as a mask, a low-density impurity, such as a n-type or a p-type low-density impurity, is ion-implanted into both end portions of the semiconductor layer 12 to form low-density source and drain regions 15-1 and 15-2.
Thereafter, the gate electrode 14 is anodized to form an anodizing layer 16 surrounding the gate electrode 14. For example, the anodizing layer is made of Al2O3, if the gate electrode is made of Al. A high-density impurity having the same conductivity as the low-density source and drain regions 15-1 and 15-2 is ion-implanted into portions of the low-density source and drain regions 15-1 and 15-2 that are not covered with the anodizing layer 16 to form source and drain regions 17-1 and 17-2.
Subsequently, a second insulating layer 18 is deposited over the whole surface of the substrate 10 and then patterned to form first and second contact holes 19-1 and 19-2. The first contact hole 19-1 is formed at a location corresponding to a portion of the source region 17-1, and the second contact hole 19-2 is formed at a location corresponding to a portion of the drain region 17-2. The second insulating layer 18 serves as an interlayer insulator.
Finally, a second metal layer is deposited on the interlayer insulator 18 and patterned to form source and drain electrodes 20-1 and 20-2. The source and drain electrodes 20-1 and 20-2 contact the source and drain regions 17-1 and 17-2 through the first and second contact holes 19-1 and 19-2, respectively. Therefore, the TFT having a lightly doped drain (LDD) structure is completed.
In order to manufacture a TFT having an off-set structure, a process of ion-implanting the low-density impurity is omitted.
The manufacture of the TFT having the LDD structure or the off-set structure requires four mask processes. A first mask forms the semiconductor layer 12. A second mask forms the gate electrode 14. A third mask forms the contact holes 19-1 and 19-2. A fourth mask forms the source and drain electrodes 20-1 and 20-2. Thus makes the manufacture of a conventional TFT very complicated and costly.
Also, the conventional method of manufacturing the TFT uses the anodizing layer 16 other than a photoresist pattern, and thus forms the LDD region in a self-aligning manner. However, an additional apparatus is necessary to anodize the gate electrode 14 and form the anodizing layer 16, leading to a very complicated manufacturing process.
In addition, since the source and drain electrodes 20-1 and 20-2 contact the high-density source and drain regions 17-1 and 17-2, a contact resistance increases, thereby degrading electric characteristics of the TFT.
FIG. 2 is a cross-sectional view illustrating a conventional organic EL display device having the TFT of FIG. 1 as a switching element. Subsequent to the process of manufacturing the TFT shown in FIG. 1, a third insulating layer 21 is formed over the whole surface of the substrate 10 so as to cover the source and drain electrodes 20-1 and 20-2. The third insulating layer 21 serves as a passivation layer. The passivation layer 21 includes a through hole 22 at a location corresponding to a portion of either of the source and drain electrodes 20-1 and 20-2. In FIG. 2, the through hole 20 is formed on a portion of the drain electrode 20-2.
Then, a transparent conductive material is deposited and patterned to form a pixel electrode 23. The pixel electrode 23 contacts the drain electrode 20-2 through the through hole 22 and serves as an anode electrode 23.
Subsequently, a planarization layer 24 is formed over the whole surface of the substrate 10. The planarization layer 24 has an opening portion 25. The opening portion 25 exposes a portion of the anode (pixel) electrode 23.
Next, an organic EL layer 26 is formed on the exposed portion of the anode electrode 23. Finally, the organic EL display device is completed when a cathode electrode 27, which covers the organic EL layer 26.
In order to manufacture the organic EL display device shown in FIG. 2, three mask processes are required in addition to the four mask processes required to manufacture the TFT. A fifth mask forms the through hole 22. A sixth mask forms the anode electrode 23. A seventh mask forms the opening portion 25. These additional processing operations and time complicate the manufacturing process of the organic EL display device. As a result, the manufacturing yield is low, and the production cost is high.